Cluster Verification (cluvfy or CVU) detects whether DNS is configured properly by looking up an unknown host and expecting the command to finish in less than 15 seconds.
$ time nslookup unknown-not-reachable-node
DNS LOOKUP From all server (rac1,rac2) nsllokup rac1,rac2,rac-scan and rac1.testonline.com,rac2.xxxx,rac-scan.xxxx result
REVERSE LOOKUP From all server (rac1,rac2) nsllokup 10.77.1.171 and 10.77.1.188
TIME RESULT NEAR ZERO
[oracle@rac2 ~]$ time nslookup rac2
The error is returned because node exceeded a response time for an UNREACHABLE node not a REACHABLE node.
time nslookup unknown-not-reachable-node